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Context. A molecular survey recently performed ahead of HH~2 supports theidea that the observed molecular enhancement is due to UV radiation from the HHobject. Aims. The aim of the present work is to determine whether all HHobjects with enhanced HCO$^+$ emission ahead of them also exhibit the sameenhanced chemistry as HH~2. We thus observed several molecular lines at severalpositions ahead of five Herbig-Haro objects where enhanced HCO$^+$ emission waspreviously observed. Methods. We mapped the five Herbig-Haro objects using theIRAM-30 m. For each position we searched for more than one molecular species,and where possible for more than one transition per species. We then estimatedthe averaged beam column densities for all species observed and also performedLVG analyses to constrain the physical properties of the gas. Results. Thechemically richest quiescent gas is found ahead of the HH~7-11 complex, inparticular at the HH~7-11 A position. In some regions we also detected a highvelocity gas component. We find that the gas densities are always higher thanthose typical of a molecular cloud while the derived temperatures are alwaysquite low, ranging from 10 to 25 K. The emission of most species seems to beenhanced with respect to that of a typical dense clump, probably due to theexposure to a high UV radiation from the HH objects. Chemical differentiationamong the positions is also observed. We attempt a very simple chemicalanalysis to explain such differentiation.Comment: Accepted by Astronomy and Astrophysics; 17 pages, 8 figure

An observational survey of molecular emission ahead of Herbig-Haro objects